1. Field of the Invention
The invention relates to liquid atomization, in which atomizing gas is heated by indirect heat exchange with the hot liquid to be atomized. More particularly, the invention relates to a liquid atomization apparatus and process in which atomizing steam is heated to a superheat temperature and a high velocity, by indirect heat exchange with the hot liquid to be atomized. This is useful for atomizing the hot feed oil in an FCC process.
2. Background of the Invention
Atomizing hot, relatively viscous fluids at high flow rates, such as the heavy petroleum oil feeds used in fluidized catalytic cracking (FCC) processes, or fluid cat cracking as it is also called, is an established and widely used process in the petroleum refining industry, primarily for converting high boiling petroleum oils to more valuable lower boiling products, including gasoline and middle distillates such as kerosene, jet and diesel fuel, and heating oil. In an FCC process, the preheated oil feed is mixed with steam or a low molecular weight (e.g., C4xe2x88x92) gas under pressure, to form a two phase fluid comprising the steam or gas phase and the liquid oil phase. This fluid is passed through an atomizing means, such as an orifice, into a lower pressure atomizing zone, to atomize the fluid into a spray of oil droplets which contact a particulate, hot cracking catalyst. Feed atomization is initiated immediately downstream of the atomizing orifice or means, and may continue into the downstream riser reaction zone. Steam is more often used than a light hydrocarbon gas, to reduce the vapor loading on the gas compression facilities and the downstream products fractionation. With the trend toward increasing the fraction of the very heavy and viscous residual oils used in FCC feeds, more and hotter steam is needed for atomization. However, many facilities have limited steam capacity and the steam is typically saturated, which constrains their ability to effectively process heavier feeds.
The invention relates to a fluidized cat cracking (FCC) process in which the hot feed oil is atomized with an atomizing gas, and wherein at least a portion of the atomizing gas has been heated by indirect heat exchange with the hot oil feed. The heat exchange takes place upstream of the atomizing means, in at least one heat exchange means which may comprise, for example, a heat conductive apparatus or body having a plurality of fluid passage means therein, with each fluid passage means having at least one fluid entrance and exit, to permit the gas and the hot oil to flow separately into and through, in indirect heat exchange, during which the hot oil heats the gas. By atomization is meant that the liquid feed oil is formed into a spray comprising discrete and dispersed, small drops or droplets of the oil. Atomization is achieved by conducting the fluid through at least one atomizing means, into a lower pressure atomizing zone. When more than one atomizing means is used, they may be in a series or parallel flow arrangement, preferably parallel. The heated atomizing gas preferably comprises steam, which may or may not be in admixture with one or more other gases, such as hydrocarbon gases and vapors. Thus, the term xe2x80x9csteamxe2x80x9d as used herein is not meant to exclude the presence of other gases in admixture with the steam. However, the atomizing gas preferably comprises at least 95 volume % steam and more preferably all steam. In the practice of the invention, the steam is heated to a superheat temperature and, in a preferred embodiment, the superheated steam exits the heat exchange means and is injected into the flowing, hot, oily fluid at a high velocity. By high velocity is meant a steam Mach number of preferably greater than 0.5, more preferably greater than 0.8, and still more preferably greater than 0.9. The hot oil flowing through the heat exchange means may be a single-phase fluid comprising the hot feed oil or a two-phase fluid comprising gas, as in preferably steam, and the hot oil. Hereinafter, the term xe2x80x9cfluidxe2x80x9d as used herein is meant to include both a single liquid phase, and a two-phase mixture comprising a gas phase and a liquid phase. The superheated steam, preferably at a high velocity, is injected into the flowing fluid to increase the surface area of the liquid phase. Increasing the velocity reduces the amount of steam required and increases the kinetic energy available for increasing the liquid surface area (e.g., e=mv2), which is ultimately manifested by smaller droplet sizes of the atomized oil spray. The superheated steam may be injected into the flowing hot fluid either inside, outside, upstream or downstream of the heat exchange means. The superheated steam injection results in either (i) a two-phase fluid comprising the steam and hot feed oil or (ii) a two-phase fluid in which the surface area of the liquid phase has been increased. That is, if the hot fluid into which the steam is injected is a single-phase liquid, injecting the steam into the liquid produces a two-phase fluid comprising a steam phase and a liquid phase. If the fluid into which the steam is injected is a two-phase fluid comprising steam (or gas) and the hot liquid oil, injecting the steam into the fluid increases the surface area of the liquid phase of the fluid. The two-phase fluid is passed into and through an atomizing means and into a lower pressure atomization zone, in which the steam expands and forms a spray comprising atomized droplets of the oil. The atomizing means typically comprises a pressure reducing and velocity increasing orifice, as is known, but it may also comprise a pressure reducing and velocity increasing region or zone, just upstream of the lower pressure atomizing zone, in which the steam expands sufficiently to form the spray of oil droplets. The atomizing means may or may not comprise part of the heat exchange means, as is described in detail below. If it comprises part of the heat exchange means, it will typically be located proximate to its fluid exit. In another embodiment, all or a portion of the superheated steam formed in the heat exchange means may be directed as xe2x80x9cshock steamxe2x80x9d into the two-phase fluid, as it exits the atomizing means and enters the lower pressure atomizing zone, to provide a more uniform drop size distribution of the atomized oil.
In an FCC process in which at least a portion of the atomizing steam is heated to a superheat temperature according to the practice of the invention, the hot feed oil will typically be injected or mixed with a portion of the atomizing steam to form the two-phase fluid, prior to being injected with the superheated steam produced in the heat exchange means. This will typically occur upstream of the heat exchange means. A portion of this prior or upstream steam may be superheated, but is more typically all saturated steam. In one embodiment, the heat exchange means may include atomizing means such as an orifice. In another embodiment it will include means for mixing the two-phase fluid formed upstream to increase the surface area of the liquid feed oil phase. In the practice of the invention, the temperature drop incurred by the hot oily fluid flowing through the heat exchange means, as it heats the steam to a superheat temperature, will be typically less than 6xc2x0 C. If saturated steam is passed into the heat exchange means, then passage of the steam through this means superheats the steam and this superheated steam is injected or impacted into the flowing hot fluid. If superheated steam is passed into the heat exchange means, its superheat temperature will be increased. In either case, the superheated steam heated or formed in the heat exchange means is directed into the flowing hot fluid as atomizing gas. Both the heat exchange and atomizing means will typically comprise part of a feed injection unit, which sprays the hot, atomized oil droplets into a cat cracker reaction zone, in which they contact hot catalyst particles which catalytically crack the hot oil into more valuable, and generally lower boiling, material. The injection unit will generally comprise a feed conduit in which a steam sparger is located, to form a two-phase fluid comprising the hot oil feed and the steam. The conduit feeds this two-phase fluid into the heat exchange means and the superheated steam formed in this means is injected into the flowing fluid to increase the surface area of the liquid phase. While a single-phase liquid fluid may be passed into the heat exchange means, in an FCC process it will more typically be a two-phase fluid comprising steam and the liquid feed oil. In an embodiment in which the heat exchange means also mixes the flowing fluid, the fluid will be a two-phase, steam-continuous fluid comprising a steam phase and the liquid feed oil phase. In any case, a two-phase fluid is formed before, or as a consequence of, the superheated steam injection and is preferably steam-continuous when passed through the atomizing means. The two-phase fluid is passed into and through atomizing means into a lower pressure atomizing zone in which the steam expands and the fluid is atomized to form a spray of oil droplets. A spray distribution means or tip, is preferably used to shape the spray of liquid droplets into the desired shape and is typically located proximate the downstream end of the injection unit. This spray distribution means is located downstream of the atomizing means or its upstream entrance may comprise atomizing means.
In the practice of the invention, the fluid pressure upstream of the downstream side of the atomizing means is higher than that in the atomizing or expansion zone(s). In an FCC process, the pressure of the fluid in the injector is above that in the atomizing zone which, in an FCC cat cracking reaction process either comprises, or opens into and is in direct fluid communication with, the cat cracking reaction zone. This reaction zone typically comprises a riser, as is known. Superheating the steam so that it is injected into the fluid at a high velocity will produce a smaller Sauter mean droplet diameter of the resulting atomized liquid, even with a very low fluid pressure drop (e.g., xcx9c69 kPa) through the atomizing means or orifice. Injecting high velocity steam at a Mach number greater than 0.5 into the fluid, reduces the amount of steam needed for atomization, without increasing the size of the atomized liquid droplets. Vaporization of the feed in the shortest time possible leads to greater amounts of useful crackate products. Feed vaporization is a function of many factors, including the droplet size of the atomized feed liquid and the shape and uniformity of the atomized spray of liquid droplets.
In a broad sense, the process comprises an atomization process in which a hot fluid, comprising the liquid to be atomized flows through a heat exchange means, in indirect heat exchange with an atomizing gas, to heat the gas. In the context of the invention, the term xe2x80x9cgasxe2x80x9d is meant to include steam and/or any other gaseous material suitable for use as an atomizing fluid, such as for example, C4xe2x88x92 hydrocarbon vapors, nitrogen and the like. However, in an FCC process it is typically all steam. The heated atomizing gas is injected at high velocity into the flowing hot fluid, to assist in atomizing the liquid in the fluid, into a spray of small droplets. As discussed, this fluid is atomized, by passing it through at least one atomizing means, such as an orifice and into a lower pressure atomizing zone. The fluid flowing through the heat exchange means may be a single phase of the liquid to be atomized or a two-phase fluid comprising the liquid and an atomizing gas. The fluid will comprise a two-phase fluid, and most preferably a gas-continuous, two-phase fluid, when passing through an atomizing orifice. This two-phase fluid is formed either before injecting the superheated steam into the fluid, or as a consequence of the superheated steam injection. In either case, the fluid will comprise a gas-continuous, two-phase fluid after the superheated steam injection. The pressure in the heat exchange means and upstream of the atomizing means is greater than that in the downstream atomizing zone. In a more detailed embodiment with respect to a typical FCC process, the invention comprises the steps of:
(a) injecting atomizing steam into a flowing, hot, liquid FCC feed oil under pressure, to form a two-phase fluid comprising the hot oil and steam;
(b) passing steam and the hot, two-phase fluid formed in (a) through separate conduits in a heat exchange means, in which the flowing hot fluid heats the steam to a superheat temperature, by indirect heat exchange with the fluid;
(c) injecting superheated heated steam formed in (b) into the hot fluid to increase the surface area of the liquid phase and form a steam-continuous two-phase fluid;
(d) passing the steam-continuous fluid through at least one atomizing means into at least one lower pressure atomizing zone to at least partially atomize said fluid and form a spray comprising droplets of said feed oil.
The spray may be formed in or near a cat cracking zone, or it may be conducted into the cat cracking reaction zone.
Further embodiments include: (i) contacting the spray with a particulate, hot, regenerated cracking catalyst in the reaction zone at reaction conditions effective to catalytically crack said feed oil and produce lower boiling hydrocarbons and spent catalyst particles which contain strippable hydrocarbons and coke; (ii) separating said lower boiling hydrocarbons produced in step (e) from said spent catalyst particles in a separation zone and stripping said catalyst particles in a stripping zone, to remove said strippable hydrocarbons to produce stripped, coked catalyst particles; (iii) passing the stripped, coked catalyst particles into a regeneration zone in which the particles are contacted with oxygen at conditions effective to bum off the coke and produce the hot, regenerated catalyst particles, and (iv) passing the hot, regenerated particles into the cat cracking zone.